I've learned my fair share of languages, at different levels of proficiency, mostly because of necessity.

I'm native in 2, fluent in another one. I learned 3 more by living in a foreign country, speaking those languages, where I can use them to communicate with colleagues or friends, but where I would benefit a lot from learning them for a few more years to reach C level (I'm probably around a B2 level for them). I finally have small bits of knowledge in others that I learned at school as a kid, but I mostly forgot it.

Two of the ones that I learned by living extensively in the other country, when over 30, I approached them mostly as follows:

• Learn the basic grammar patterns
• Learned tons of vocabulary. When I say tons, I really mean tons. For that, I used spaced repetition (Memrise decks mostly, but also some other tools)
• Only after doing the first two steps did I start using it with others in real life. This was very important to consolidate the vocabulary I had learned without context. But without that big database of vocab, there was no point in trying to have discussions. You need a lot of vocab to communicate; you can get by with a small amount of grammar. For most languages. This goes contrary to what is often thought in schools, I believe.
• All of that, I mixed it in with Italki, Verbling, Duolingo, in-person classes, online classes, etc. Just maximize the input. Italki-type language partners, where you pay your partner, is much more efficient than the classic in-person language exchange because you don't waste time teaching your own language (or even getting distracted in a common language).

The third one that I learned while in another country: I was much younger (18 years old), and I was living in a host family where English was not an option. With the level of brain plasticity at that age, the constant exposure at a local school, learning happened quite naturally. Also, tech tools were much more limited at the time, so I could not apply the approach I outlined above efficiently.

I used to run a language learning blog with more useful, language-specific tidbits, but sharing it would probably doxx me. I'll have to see if I can recycle some of it and render it authorless by putting it through an LLM.